Method of converting roasted lead ores and bag-house dust directly into lead salts



Patented July 4;, 1933 stares rarest orrica LEO CURTTLN, E CRANBURY, NEW JERSEY METHDID 0E CONVERTING ROASTED LEAD GEES AND BAG-HOUSE DUST DIRECTLY INTO-LEAD SALTS No Drawing.

. matter, first with a solution of lead acetate to dissolve the basic lead or lead oXid content, andsecond, with a solution of calcium acetate or ammonium acetate to dissolve the lead sulfate content.

The present invention relates to an improved method of treating such mixtures con taining lead oxid, lead sulfate and music lead sulfate, and is particularly designed and suitable for the treatment of bag-house fume or bag-house dust for converting its lead content directlyinto valuable compounds without at any point in the process producing metallic lead. The invention will be described and illustrated in connection with the treatment of lead fume, but, as will be apparent, is not limited thereto. I

In the smelting of ores it commonly occurs that a small portion of the lead, say 1 to 4%, is volatilized by the heat of the smelting operation and is condensed and collected as fume in what are known as bag-houses. The coarser portions of thefume are called baghouse dust and the finer portions bag-house fume. The composition of this dust and fume varies somewhat at different smelters, but in all cases lead sulfate is avery important constituent and lead in-other forms, such as lead oXid or basic lead sulfate, is generally present in substantial quantities. An analysis of the fume from a large smelter in the St. Louis district shows that it contains about of lead calculated as lead metal, and that 77% of this lead is present as sulfate and 23% in basic form. Analyses of the byproduct from other smelters may show a somewhat higher percentage of impurities, and also moderate variations in the ratio of basic lead to lead sulfate.

A characteristic of all bag-house sublimates is that they are very largely free from the non-volatile impurities present in Application filed May 3, 1932. Serial No. 609,044.

the original ore, such as alumina, silica and more desirable material to use as a raw material than any lead ore, even when such lead ore has been roasted so that its lead is present as OXld, basic sulfate and sulfate.

in the extraction of lead from fume, lead acetate solution alone is valueless because of its inability to dissolve more than traces of lead sulfate. Calcium acetate is a better solvent for the lead sulfate, but has the disadvantage of becoming saturatedwith soluble lead compounds when the solution contains only two or three per cent of such lead compounds, and furthermore calcium acetate is completely unable to dissolve the more refractory portions of the fume. Sodium acetate is even, less effective than calcium acetate and potassium acetate is value less for the purpose, since it forms an insoluble double sulfate with lead.

It is well known that ammonium acetate is a solvent for lead sulfate and even shows some solvent effects on basic lead sulfate. Its principal disadvantage is that, as ordinarily used, it does not dissolve sufficient lead sulfate to be commercially advantageoumand by bringing the sulfate ion into solution along with the lead, it gives a solution from which it is very difficult to prevent the precipitationof lead as sulfate during subsequent operations with the solution. I have found that, if soluble alkaline earth metal salt, such as an alkaline earth metal acetate, be dissolved in the ammonium acetate solution, in the proper proportion, greatly in creased dissolving of the leadcontent of the fume is obtained, and most of the sulfate is left in the residue as alkaline earth metal sulfate. Furthermore, by utilizing the principles of counter-current washing, it is possible to extract practically all of lead present, leaving a residue of the impurities originally present in the fume, together with crystalline alkaline earth metal sulfate. This residue may be readily separated by mechanical or chemical means, into alkaline earth metal sulfate, 011 the one hand, and the cadmium and other compounds previously mentioned, on the other hand, these latter being then worked up in any known manner for other valuable derivatives.

As an alkaline earth metal salt to reinforce the ammonium acetate, I prefer to use calcium acetate because of its cheapness. Other calcium salts may be used, even calcium hydroxid, which may be dissolved in the ammonium acetate or mixed dry with the fume before extraction. In any event, it is desirable to have the solution or the dry fume contain enough calcium salt to convert all of the sulfate present into gypsum.

The advantage of using the calcium salt is that it converts the sulfate present in the fume into an insoluble compound which remains behind in the residue. The resulting solution, under these conditions, contains lead in abundance, but only a small quantity of sulfate ion, and the tendency of lead to precipitate as lead sulfate is, therefore, very greatly reduced. Careful experimentation on different materials has shown that it IS possible to dissolve 2 times as much lead from fume by the use of ammonium acetate in conjunction with calcium acetate as would be obtained if only ammonium acetate were used.

I prefer to use a solution of ammonium acetate ranging from 25 to 35% in concentration and containing as much calcium acetate as experience shows is required to com pletely react with the sulfate ion present. For example, 100 cc. of a 30% solution of ammonium acetate, containing 5% by weight of calcium ion as either acetate or hydroxid, is capable of dissolving from fume or from lead sulfate as much as 20 grams of lead calculated as metallic lead. As stated above, the proportion of calcium ion may be varied slightly, as experience will indicate. The ammonium acetate solution may be used with good results under neutral, basic, or acid conditions. I find it most convenient to use when it contains a small excess of free acid.

The ammonium acetate solution is preferably used hot, temperatures ranging from 90 to 100 C. After dissolving the lead, it is readily separated from the residue by filtration or decantation. The clear filtrate, which is usually water white in color, may then be used to prepare lead salts.

In the preparation of lead chromate or lead arsenate, a soluble chromate or arsenate is added to the lead solution under appropriate conditions. An immediate precipitation of the desired product occurs. In the preparation of white lead or lead basic carbonate it is usually necessary to make the solution alkaline either by using lime in some way in the original extraction or by adding lime or other alkali to the lead-containing ammonium acetate solution. This solution readily dissolves calcium hydroxide and in so doing regenerates thecalcium acetate which is used in the original extracting solution. Carbon dioxid is then introduced into the solution, and lead basic carbonate is thrown down. The precipitate is then filtered out and the solution is now in condition for a second extraction of fume.

The presence of ammonium salts in a lead. containing solution from which lead basic carbonate is to be precipitated is of great advantage. When alkali, such as sodium by droxide or calcium hydroxide, is added to such a solution, the pH of the solution does not increase nearly as much as if the ammonium salts were absent. A great deal of the added base is transformed into ammonium hydroxide by interaction with ammonium acetate, thereby replacing a strong base by a much weaker one. At the concentrations used, ammonium hydroxide is only 1/80 as strong as the corresponding quantity of cal cium hydroxide and 1/200 as strong as the equivalent of sodium hydroxide. This holds the pH Within a range favorable for the precipitation of lead basic carbonate, while, in the absence of ammonium salts, the solution becomes so basic that the resultant precipitate of lead is more basic than is desirable.

I claim:

1. Method of recovering the lead content of material containing lead sulfate which comprises subjecting the material to the solvent action of a solution of ammonium acetate in the presence of a soluble alkaline earth metal compound.

2. Method as defined in claim 1 in which the lead sulfate-containing material contains also lead oxid and lead basic sulfate.

3. Method as defined in claim 1 in which the lead sulfate-containing material is lead fume.

4. Method as defined in claim 1 in which the alkaline earth metal compound is calcium acetate.

5. Method as defined in claim 1 in which the ammonium acetate solution is acidified.

6. Method as defined in claim 1 in which sulfate present in the lead sulfate-containing material to the formation of an insoluble alkaline earth metal sulfate.

8. Method as defined in claim 1 in which the lead sulfate-containing material is treated with ammonium acetate solution by countercurrent washing.

9. Method as defined in claim 1 in which tion, adding calcium hydroxid to the solution, treating the solution with carbon dioxid to precipitate its lead content, and using the resulting solution for extracting more lead fume.

In testimony whereof, I aflix my signature.

LEO P. OURTIN. 

